RE: Overheating MOT - dual 833A VTTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mccauley, Daniel H by way of Terry Fritz <teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>


Great information John.  Its a lot to think about.  I think i have to
rebuild a few things on my VTTC to make
more easily adjustable such as an adjustable grid coil, etc... so that I can
try different things in a timely fashion to
see what works and what doesn't.  Regardless, this is very quickly becoming
my most favorite coil so far.  Its a lot of fun
to play with and watch.  Plus its relatively quiet as well!

Dan



 > Dan,
 >
 > I've seen this effect at times in my coils.  Sometimes if the variac
 > level is too high, and depending on the circuit parameters, the grid
 > voltage may momentarily rise higher than the plate voltage during a
 > part of the RF cycle.  The grid then behaves like
 > the plate in a sense, and draws too much
 > current.  This can destroy the grid, and will reduce the spark output.
 > It usually creates a very raspy sound when it does this, and the
 > spark will become "fuzzy" (not sword-like).  In general a raspy sound
 > is "not good".  This can possibly be detected better in non-staccato
 > mode.  The raspiness can be reduced by adjusting the grid resistance,
 > and the amount of grid coil feedback.  Another possibility is that the
 > tuning is not at it's best, although you probably tried
 > re-tuning.  If the
 > tuning is not correct, the spark length will peak before full
 > power is
 > applied.
 > The best tuning for short sparks will be somewhat different than the
 > best tuning spot for long sparks.  Tuning affects the degree of
 > sword-like-ness too.  VTTC's can be sensitive to the
 > input current waveform also.  For example when I ran my coil from
 > a powerful plate transformer, it was difficult to get very
 > straight sparks.
 > Simply by installing the MOT in it's place, the sparks became
 > sword-like.
 > I think this is because the poor quality of an MOT tends to distort
 > the input waveform, and actually improved the spark appearance.
 > This may be affected by the number of MO caps in the level-shifter
 > also.  I obtained the best results with only two MO caps.  All the
 > things I mentioned may interact also, so a combination of adjustments
 > may be needed.  Sometimes a change in the coupling may give
 > some benefit also.  The fact that you're getting 28" at only 80% input
 > voltage also shows that your tank impedance is lower than what I
 > used in my coil.  This causes the coil to draw a heavier current at
 > a lower voltage.  This can often make the vacuum tubes get too red
 > for comfort.  If your's are OK in that regard, then you may
 > be obtaining
 > a benefit from the somewhat larger primary and secondary that you
 > are using compared to my coil.  In any case this may cause some
 > part of the circuit to show some unexpected non-linearity, etc., and
 > cause the loss of the swordlike appearance, as you go over 80%
 > input voltage.  These are just some things to consider.  In general,
 > it seems to get harder to maintain the sword-like spark appearance
 > as the spark increases in length.
 >
 > I should probably mention that there are two types of sword-like
 > sparks.  In one case, there are multiple sparks which are pointy and
 > straight.  This can be seen at a 60 PPS rate, and at a somewhat
 > slower rate.  At slower staccato rates however, the spark can
 > create just a single straight "sword-blade" without any branching.
 > This blade will waver and sway around some, but will maintain
 > it's single blade-like appearance.  This type of spark can be seen
 > at my website.  Photos of VTTC sparks can sometimes show
 > this appearance when the appearance actually cannot be seen by the
 > human eye.  In the case of my sword-like photos, it's what the eye
 > sees.  Just thought I'd mention this because it's not something I've
 > discussed a lot on the list.
 >
 > My 36" spark VTTC was never able to give either type of swordlike
 > spark for some
 > reason.  It may have had something to do with the larger secondary
 > wound with many turns of thin wire.  VTTC's seem to like thicker
 > secondary wire with fewer turns.  In some tests in which I installed
 > a much shorter secondary with fewer turns of the same wire (for a
 > lower resistance), the sparks did become more sword-like.  This
 > shorter secondary was 7" x 9" tall and gave 35" sparks before a
 > massive flashover destroyed the secondary, and damaged the
 > primary.
 >
 > In any case, you're getting nice results overall.
 >
 > John
 >
 >
 > >This evening i got my VTTC to consistently hit the 28" mark.
 >  I was really
 > >pscyched.  This was at 80% input voltage.  Any additional
 > increase in input
 > >voltage caused the swordlike appearance of the streamers to
 > decrease and
 > >therefore decreasing overall length.  BTW, I'm currently using (5) MO
 > >capacitors for a total of about 4uF.  I'll be posting
 > pictures later this
 > >evening once I download them off my camera.
 > >
 > >Dan
 > >
 > >
 >
 >
 >
 >